Many different types of apparatus are known for the removal of light reject from fiber suspensions. For example, U.S. Pat. No. 4,634,521 discloses a screen in which so-called plastics separation is arranged inside a rotor. However, the described pressurized apparatus is not capable of reliably removing, for example, particles of expanded polystyrene as such particles are compressible and are, therefore, due to the pressurized atmosphere within the apparatus, compressed and pass through the apertures of the screen to the accept flow.
In addition, light weight compressible material can also successfully be removed from fiber suspensions with known vibrating screen and drums. Also, curved screens can be used for this purpose (U.S. Pat. No. 4,333,572) as well as an inverted cyclone as disclosed in Canadian Patent 1,203,778. The above disclosed devices have been used in the paper manufacturing process for the separation of light weight waste material, so-called light reject, by passing the stock flow through the separator which, of course, means that the number and/or the size of the devices have increased which also correspondingly increased the installation, operation and maintenance costs thereof.
Typically in the paper making process, as much gas as possible is withdrawn from the fiber suspension before the suspension is fed onto the forming wire of the paper machine. Most commonly used for the removal of gas from the fiber suspension is a device wherein the fiber suspension is initially conveyed to a tank in which a negative pressure is maintained. The level of stock in the tank is kept constant either by providing an overflow over a weir or partition wall, thus separating the stock to be fed to the headbox from the stock which is returned to the circulation (U.S. Pat. No. 4,419,109), or by regulating with a transducer the pumping of the stock to the tank, or by a combination of both. The negative pressure prevailing in the tank, the amount of which is adjusted to be close to the boiling pressure of the suspension, causes the gases in the suspension both, in the form of bubbles and in a dissolved state, to be separated from the suspension whereby the gases are readily removable with a vacuum generating device. It is typical of the first mentioned apparatus that the fiber material passing the overflow is recirculated by directing the flow to the wire pit or to a corresponding location at the inlet of the screening device, usually hydrocyclones, which precede the air removing device in the process. Frequently large amounts of light plastic rejects or the like collect at the surface of the stock suspension in the wire pit and tend to accumulate in the short circulation. This is, of course, less detrimental than the light plastics flowing to the headbox and further onto the wire where they cause holes in the paper manufactured. However, there is no disclosure in the prior art for removing light reject from fiber suspensions in connection with the removal of air therefrom.
Today, the fiber suspensions generally contain more and more light substances which are compressible in a way that they cannot be reliably removed with pressurized perforated or slotted screens. Such light substances will therefore accumulate in the process and, in the long run, will cause problems in the end product. It is, therefore, desirable to provide a phase in the suspension treatment process in which the light material can be removed. Since the light reject contains mainly compressible material a proper removal of the reject can only be achieved at a point where the suspension is not under pressure. For example, in the degassing system of a paper machine short circulation light material present in the fiber suspension will rise to the surface and pass via an overflow to the wire pit and, more generally, to the short circulation.